Plasma generators are generally known as ion sources, electron sources or plasma sources and are used as an ion source, for example, in ion engines for space engineering. The plasma generator according to the invention is a high-frequency plasma generator. When this plasma generator is used in a high-frequency ion engine, a working fluid, also called fuel or auxiliary fluid, that is introduced into the ionization chamber is ionized using an electromagnetic alternating field and is then accelerated for generating thrust in the electrostatic field of an extraction lattice system provided at an open side of the ionization chamber. The ionization takes place in the ionization chamber which is surrounded by a coil. A high-frequency alternating current flows through the coil. The alternating current generates an axial magnetic field in the interior of the ionization chamber. This magnetic field, which varies with respect to time, induces a circular electric alternating field in the ionization chamber.
This electric alternating field accelerates free electrons so that the latter can finally absorb the energy required for the electron impact ionization and atoms of the fuel are thereby ionized. The ions are either accelerated in the extraction lattice system or they recombine at the walls with electrons. The released electrons are either accelerated in the field or may themselves absorb the energy required for the ionization, or collide with the walls of the ionization chamber and recombine there.
In principle, the ionic current generated in an ion source, for impressing a defined energy, can be used for many different processes. For example, when used as an ion engine the acceleration of the ions is utilized for generating thrust according to the recoil principle.
In conventional ion sources, particularly in conventional ion engines, only a small number of ions find their way to the extraction lattice system, while the majority of the generated ions recombine on the walls of the ionization chamber. Only those ions that reach the extraction lattice system, when used as an ion engine for generating thrust or when used as a general ion source, will be available for the utilization in other processes. Of the total supplied electric power, so far, only approximately 5% to 20% of the electric power can be converted for this utilization of ions in a general ion source or in an ion engine. The remaining supplied electric power is, for the most part, converted to heat and to radiation by the recombination of the ions on the wall of the ionization chamber. A minimal ionization energy Wi is required for generating an ion. In the case of the recombination on the walls, Wi is released in the form of heat and radiation and is therefore unavailable for a further ionization or for the utilization by acceleration in the extraction lattice. The wall recombination is therefore the largest loss factor during the high-frequency ionization.
Exemplary embodiments of the present invention provide a plasma generator that reduces the power loss occurring by recombination of the ions and/or electrons on the wall of the ionization chamber.
One exemplary aspect of the present invention provides a plasma generator comprising a housing surrounding an ionization chamber, at least one working-fluid supply line leading into the ionization chamber, the ionization chamber having at least one outlet opening, and at least one electric coil arrangement surrounding at least one area of the ionization chamber. The coil arrangement is electrically connected with a high-frequency alternating-current source (AC) which is constructed such that it applies a high-frequency electric alternating current to at least one coil of the coil arrangement. A further current source is provided which is constructed such that it applies a direct current or an alternating current of a frequency lower than that of the current supplied by the high-frequency alternating current source (AC) to at least one coil of the coil arrangement.
This plasma generator reduces the power loss occurring by recombination of the ions and/or electrons on the wall of the ionization chamber.
The power loss reduction is achieved using a further current source or voltage source in addition to the known high-frequency alternating current. This current source or voltage source is designed such that a direct current or an alternating current of a frequency lower than that of the current supplied by the high-frequency alternating current source is applied to at least one coil of the coil arrangement. The direct current or alternating current of a lower frequency thereby additionally fed into the coil arrangement superposes on the magnetic high-frequency alternating field a magnetic direct field fraction or at least a fraction of a lower-frequency magnetic alternating field. Although aspects of the invention may be described using current sources is described, voltage sources may also be employed.
The Lorentz forceF=q(v×B)wherein the charge is q, the velocity is v and the magnetic flux density is B, acts upon moving charge carriers in the magnetic field. The direct current fraction superposed on the magnetic alternating field or also the fraction of the lower-frequency alternating current superposed on the high-frequency electromagnetic alternating field has the effect that the charge carriers (electrons and ions) inside the coil and thus inside the ionization chamber are forced into orbits or spiral paths in the magnetic field. Such an orbital motion or spiral path motion of the electrons in the magnetic field reduces their movement in the direction of the wall (the so-called confinement). Since the movement of the electrons and ions from the interior of the ionization chamber to the walls and to the extraction lattice system takes place in an ambipolar manner, the flux of the ions to the walls is also correspondingly reduced. In this manner, the probability of a collision of charge carriers with the wall and thus the recombination of ions and/or electrons on the walls is clearly reduced with the plasma generator according to the invention. The ions that move in the desired direction—which, in the case of an ion engine, is the direction parallel to the longitudinal axis toward the extraction lattice system—move parallel to the magnetic lines of flux and are not hindered in their movement there by the additionally applied magnetic direct field or alternating field of a lower frequency.
The direct current, or alternating current of a lower frequency, superposed on the high-frequency alternating current flowing through the coil arrangement, is selected such that it is sufficient for obtaining a magnetic field of a desired level in the ionization chamber. The gas in the interior of the ion source, thus, in the ionization chamber, represents plasma. When an inhomogeneous magnetic field is superposed on a plasma, the plasma will move in the direction of the magnetic field that is becoming weaker (gradient drift). While the geometry of the coil arrangement is designed correspondingly, it becomes possible to move the charge carriers in the plasma as a result of gradient drift increasingly in the desired direction, for example, in the direction toward the extraction lattice system.
According to exemplary embodiments of the present invention, it becomes possible to reduce the wall losses in the ionization chamber of plasma generators, such as ion sources, particularly of ion engines, without having to change the basic design of the previously known ion sources or ion engines. In addition, the invention can be used for controlling the distribution of the plasma density in the ionization chamber. Together with the design of the ionization chamber and of the cooling arrangement, it can also be used for minimizing the wall losses. Furthermore, in the case of a plasma generator according to the present invention, the homogeneity of the plasma in the ionization chamber can be optimized when the design of the ionization chamber and of the coil arrangement is appropriate. The invention can also be used for increasing the plasma density in desired areas of the ionization chamber. It can also be used for increasing the electron flow from an electron source.
Further preferred and advantageous development characteristics of the plasma generator according to the invention are disclosed herein. The plasma generator may be constructed as a plasma source, as an electron source or as an ion source.
In one aspect of the present invention, an accelerating device for ions formed in the ionization chamber or electrons is provided in the area of the outlet opening.
When the accelerating device is an ion source, it can have an electrically positively charged lattice and a negatively charged lattice which, in the outflow direction of the ions from the ionization chamber, is situated behind the positive lattice. The accelerating device accelerates the ions forming in the ionization chamber into a direction rectangular to the plane of the lattices out of the ionization chamber and thus causes an ion ejection from the ion source. The lattices form an extraction lattice system. In the case of an electron source, the sequence of the lattices and thus the polarity will be transposed.
Such an ion source can be a component of an ion engine.
In another aspect of the present invention, an electron injector is provided in the downstream direction of the ionic current leaving the ionization chamber, which electron injector is aimed at the ionic current and is equipped for the neutralization of the ionic current. The electron injector can have a hollow cathode. Such a neutralization can prevent the ion source or the device connected with the ion source from becoming electrostatically charged.
In another aspect of the ion source according to the invention, a magnet arrangement is provided surrounding the ionization chamber.
Another aspect of the present invention involves the coil arrangement having a high-frequency coil which is connected to a high-frequency electric alternating voltage in order to introduce the high-frequency alternating current into the coil, and in the direct current generated by a direct voltage is also introduced directly into the high-frequency coil.
In this case, the feeding of the direct current can take place at a different location of the high-frequency coil than the feeding of the high-frequency alternating current.
As an alternative, the feeding of the direct current can take place into a direct-current coil arranged parallel to the high-frequency coil.
The direct current can be automatically controllable, and an automatic control device can be provided which automatically controls the direct current, for example, proportionately to the ionic current emerging from the ionization chamber.
The present invention also involves methods for controlling a plasma generator. In the case of this method, the plasma is subjected to an electromagnetic direct field in addition to the high-frequency electromagnetic alternating field. Instead of the electromagnetic direct field, the plasma can also be subjected to an electromagnetic alternating field with a lower frequency than that of the high-frequency electromagnetic alternating field.
In the following, preferred embodiments of the invention with additional further development details and further advantages will be described and explained in detail with reference to the attached drawings.